Y-H. Koh et al. Journal of the European Ceramic Society 24(2004)699-703 10 Cell Boundary Thickness uml Cell Boundary Thickness um Fig.5. Apparent work-of-fracture of the specimens as a function of Fig. 7. Apparent fracture toughness of the specimens as a function of cell boundary thickness. cell boundary thicknes toughness"which the original intact specimen would have experienced at that load with a given notch depth ) which represents the material,s resistance to the crack propagation, of the fibrous materials is shown in sured by the senB method using a 1. 2 mm crack depth Monolithic Si3 N4 is known to have a high fracture toughness (7.5 MPa m/2)due to the interactions between the cracks and elongated grains. Even though such elongated grains were not formed, the fracture toughness of Si3 N4 was enhanced by incorpor 1 mm ating BN interlay Like the work-of-fracture. the apparent fracture toughness increased with cell bound- y ary thickness until 37 um(10 MPa m /)and decreased thereafter. This improvement was again attributed to the crack interactions (i.e. crack deflections and delani nations) with the weak cell boundaries The advantage of the fibrous monolith is most clearly illustrated by the stress-deflection curve of the notched specimens. When the specimens had a notch on the sur- face, which corresponded to a surface crack, the fibrous monolith had a similar value of critical stress at failure as its monolithic counterpart (i.e. exhibits notch-insensitiv- ty). Moreover, the fracture did not occur in a catastrophic 1 mm fashion but in a stable manner, i.e. the load bearing capacity kept increasing after the first drop of stress Fig. 6. SEM micrographs of crack propagations of the specimens with The fracture behaviors of the fibrous monolith are cell boundary thickness of (A)18 and (B)37 um strongly dependent on the properties of cell and cell boundary, such as elastic modulus, interfacial strength, boundary thickness is attributed to these combined surface flaw, and coefficient of thermal expansion effects of crack deflections and delaminations (CTE). 2 Fibrous monoliths can show two possible Fracture toughness(Fracture toughness measurement fracture behaviors depending on the magnitude of crack by SEnB method assumes elastic behaviour up to the interactions, that is, brittle and non-catastrophic failure load of failure. Therefore, the apparent fracture tough- Considering the stored energy before fracture initiation, ness for fibrous monolithus is reported as the"normal when the strength is too high, i. e when too much strainboundary thickness is attributed to these combined effects of crack deflections and delaminations. Fracture toughness (Fracture toughness measurement by SENB method assumes elastic behaviour up to the load of failure. Therefore, the apparent fracture tough￾ness for fibrous monolithus is reported as the ‘‘normal toughness’’ which the original intact specimen would have experienced at that load with a given notch depth.), which represents the material’s resistance to the crack propagation, of the fibrous materials is shown in Fig. 7 as a function of cell boundary thickness, mea￾sured by the SENB method using a 1.2 mm crack depth. Monolithic Si3N4 is known to have a high fracture toughness (7.5 MPa m1/2) due to the interactions between the cracks and elongated grains. Even though such elongated grains were not formed, the apparent fracture toughness of Si3N4 was enhanced by incorpor￾ating BN interlayers. Like the work-of-fracture, the apparent fracture toughness increased with cell bound￾ary thickness until 37 mm (10 MPa m1/2) and decreased thereafter. This improvement was again attributed to the crack interactions (i.e. crack deflections and delami￾nations) with the weak cell boundaries. The advantage of the fibrous monolith is most clearly illustrated by the stress–deflection curve of the notched specimens. When the specimens had a notch on the sur￾face, which corresponded to a surface crack, the fibrous monolith had a similar value of critical stress at failure as its monolithic counterpart (i.e. exhibits notch-insensitiv￾ity). Moreover, the fracture did not occur in a catastrophic fashion but in a stable manner, i.e. the load bearing capacity kept increasing after the first drop of stress. The fracture behaviors of the fibrous monolith are strongly dependent on the properties of cell and cell boundary, such as elastic modulus, interfacial strength, surface flaw, and coefficient of thermal expansion (CTE).12 Fibrous monoliths can show two possible fracture behaviors depending on the magnitude of crack interactions, that is, brittle and non-catastrophic failure. Considering the stored energy before fracture initiation, when the strength is too high, i.e. when too much strain Fig. 5. Apparent work-of-fracture of the specimens as a function of cell boundary thickness. Fig. 6. SEM micrographs of crack propagations of the specimens with cell boundary thickness of (A) 18 and (B) 37 mm. Fig. 7. Apparent fracture toughness of the specimens as a function of cell boundary thickness. 702 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 699–703